The subject matter disclosed herein relates generally to combustors for gas turbine engines and more particularly to mixer assemblies for gas turbine engines.
Gas turbine engines, such as those used to power modern aircraft, to power sea vessels, to generate electrical power, and in industrial applications, include a compressor for pressurizing a supply of air, a combustor for burning a hydrocarbon fuel in the presence of the pressurized air, and a turbine for extracting energy from the resultant combustion gases. Generally, the compressor, combustor, and turbine are disposed about a central engine axis with the compressor disposed axially upstream or forward of the combustor and the turbine disposed axially downstream of the combustor. In operation of a gas turbine engine, fuel is injected into and combusted in the combustor with compressed air from the compressor thereby generating high-temperature combustion exhaust gases, which pass through the turbine and produce rotational shaft power. The shaft power is used to drive a compressor to provide air to the combustion process to generate the high energy gases. Additionally, the shaft power is used to, for example, drive a generator for producing electricity, or drive a fan to produce high momentum gases for producing thrust.
An exemplary combustor features an annular combustion chamber defined between a radially inboard liner and a radially outboard liner extending aft from a forward bulkhead wall. The radially outboard liner extends circumferentially about and is radially spaced from the inboard liner, with the combustion chamber extending fore to aft between the liners. A plurality of circumferentially distributed fuel injectors are mounted in the forward bulkhead wall and project into the forward end of the annular combustion chamber to supply the fuel to be combusted. Air swirlers proximate to the fuel injectors impart a swirl to inlet air entering the forward end of the combustion chamber at the bulkhead wall to provide rapid mixing of the fuel and inlet air.
Combustion of the hydrocarbon fuel in air in gas turbine engines inevitably produces emissions, such as oxides of nitrogen (NOx), carbon dioxide (CO2) carbon monoxide (CO), unburned hydrocarbons (UHC), and smoke, which are delivered into the atmosphere in the exhaust gases from the gas turbine engine. Regulations limiting these emissions have become more stringent. At the same time, the engine pressure ratio is getting higher and higher for increasing engine efficiency, lowering specific fuel consumption, and lowering carbon dioxide (CO2) emissions, resulting in significant challenges to designing combustors that still produce low emissions despite increased combustor inlet pressure, temperature, and fuel/air ratio. Due to the limitation of emission reduction potential for the rich burn, quick quench, lean burn (RQL) combustor, radially fuel staged lean burn combustors have become used more frequently for further reduction of emissions.
Mixer assemblies for existing radially fuel staged lean burn combustors typically include a pilot mixer surrounded by a main mixer with a fuel manifold provided between the two mixers to inject fuel radially into the cavity of the main mixer through fuel injection holes. The pilot mixer and the main mixer typically employ air swirlers to impart swirls to the air entering the mixers and to provide rapid mixing of the air and the fuel. One of the key issues associated with the development of radially fuel staged combustors is to improve the mixing in the main mixer without negatively impacting the performance of the pilot mixer at lower power operations, including combustion efficiency, emissions, stability, lean blow out, and combustor dynamics. For example, combustion air flowing from the main mixer can in some instances interact with the pilot mixer and blow out the flame in the pilot mixer causing a lean blow out. Similarly, if the stability of the pilot mixer is dependent upon the stabilization of the entire combustor, that can cause a lean blow out of the flame of the pilot mixer. Cool air from the main mixer during low power operations can also result in low flame temperatures in the combustor near the pilot mixer, increasing the potential for producing CO and UHC based on improper or incomplete combustion. In addition, another key design issue is to provide adequate cooling of the pilot mixer to avoid excessive heat that can damage the mixer assembly.